Materials that produce color change upon stimulation with energy such as light or heat may have possible applications in imaging. For example, such materials may be found in thermal printing papers and instant imaging films. Generally, known materials and compositions may require a multilayer and/or multiphase film structure and further processing to produce an image (e.g., instant imaging films such as Polaroid). In the case of facsimile and thermal head media, high-energy input of greater than 1 J/cm2 is needed to achieve good images. The compositions in multilayer/multiphase media may require control of diffusion of color-forming chemistry and further processing, and are found in separate phases and layers. Most thermal and facsimile paper coatings consist of coatings prepared by preparing fine dispersions of more than two components. The components mix and react upon application of energy, resulting in a colored material. In order to accomplish the necessary mixing, the particles need to contact across three or more phases or layers and merge into a new phase. Because of these multiple phases and layers, high energy is required to perform this process. For example, a relatively powerful carbon dioxide laser with an energy density of 3 J/cm2 at exposure times of much greater than 100 μs may be needed to produce a mark. In some instances, this high-energy application may cause damage to the imaging substrate. In many situations, it may be desirable to produce a visible mark more efficiently using either a less intense, less powerful, and/or shorter energy application.
Alternatively, marking composition for forming clear-cut marks on the surface of a molded article by laser beam irradiation can employed. The coating color-forming chemistry typically comprises a leuco-dye and phenolic developer (activator) (both of which have low solubility) dispersed as separate phases in radiation-curable polymer matrix or solvent/water-borne coating. The coating also includes an Antenna, which is a chemical species having a high extinction coefficient absorption band corresponding to an imaging laser wavelength. However, the Antenna is present in only one of the separate phases, which does not result in very efficient absorbance of laser radiation. As a result, the energy density required to mark this coating is relatively high (>0.5 J/cm2, and usually about 3 J/cm2). Use of such energy densities can result in the coating and/or the substrate under the coating being ablated or damaged. Use of such imageable coating requiring high energy densities can further complicate the use of such coatings for labeling of temperature or energy sensitive data carriers (recordable CDs, DVDs etc.). In view of the foregoing, there is a need for fast-marking coatings and color-forming materials, initiated and addressable by low-energy radiation.